1. Field of the Invention
The invention relates to a method of and an apparatus for casting near-net-shape, rectangular strands from metal and a subsequent processing thereof into metal strips according to a DSC-method (direct strip casting) in a horizontal strip casting installation, wherein the metal melt is cast with a melt feeder on a horizontally circulating metal conveyor belt with a cooled bottom, and a liquid cast product is solidified to a pre-strip on the metal conveyor belt during displacement thereof and which after leaving the metal conveyor belt is fed, mechanically tensioned, to a driver by, e.g., smooth/pinch rollers. An installation with smooth/pinch rollers is not absolutely necessary, the installation can be realized without these rollers.
2. Description of the Prior Art
Because of uneven heat dissipation during solidification process of a strip, cast according to DSC-method under inert gas atmosphere without use of a casting compound, due to the upper surface of the strip being cooled only by convection with the ambient atmosphere and by heat radiation, while the bottom is in a direct contact with a cooled metal conveyor belt, the strip deforms already during solidification, firstly, upward and then downward.
At the start of cooling, the bottom of the material layer of the strip contracts mostly due to a very large temperature gradient. The entire strip bends upwardly in the middle, which results in very high stresses in the upper layer. Because these stresses are greater than the flow stress, they are reduced during the course of solidification again by subsequent elongation (flow), whereby opposite bending of the strip middle downwardly takes place. As a result, the low layer remains elongated, and the upper one shortened.
When the strip, which is usually not guided on its upper surface, leaves the metal conveyor belt with which it is displaced, the temperature of the strip over the strip thickness equalizes due to the reduced cooling of the strip bottom, the thermal tension also equalizes. The upper shortened and the lower elongated strip regions are subjected only to the backward bending, whereby the strip arches upwardly. The produced, as a result, stresses are below or close to the yield point, so that no or a very small backward formation of the arch resulting from the flow process, can be observed. The curve upward remains and results in arching of the strip narrow sides and also in a strip head like a ski.
During a further displacement, the degree of freedom of these arches in a longitudinal direction is reduced due to the gravity force of the strip horizontally displaceable on the adjoined roller table and/or by one or more pinch or smooth rollers which follow the metal conveyor belt, and firstly the strip tip and then the entire strip is mechanically tensioned and is forced to plane-parallel displacement downwardly.
This reduction of the degree of freedom leads to a need to reduce the stresses in the strip in the non-tensioned region, and that is why the strip narrow sides arch upwardly immediately after the strip leaves the metal conveyor belt. This behavior extends backwardly up to the region of the metal conveyor belt, so that the solidified strip has no contact anymore with the metal conveyor belt, and, thus, with the cooling medium and, as a result, has a non-homogenous temperature distribution over width of the strip that has a gutter profile.
In order to deal with this problem and to prevent the backward displacement of the pre-strip profile in the casting region and to insure passing into the upstream located machine, WO 2006/066552A1 suggests to arrange a guide element at the end of a primary cooling zone and in front of a conventional secondary cooling zone. As a rule, the guide element consists of several rollers arranged above and below the pre-strip in top-to-top or in offset-to-each other condition.
With a particular arrangement of rollers, the pre-strip is displaced in a plane located above a casting line in order to absorb the elongation of the bottom of the pre-strip by the carried-out upward movement. A roller arrangement, with which the pre-strip passes through the rollers as a wave, is also possible, however, it has not been used up to now.
The drawback of the method disclosed in WO 2006/066552 A1 consists in that the guide element that follows the metal conveyor belt can only partially influence the thermal processes on the metal conveyor belt.